Preparation of biaxially oriented polyvinyl alcohol film

Abstract

Method for preparing a biaxially oriented polyvinyl alcohol film by extruding a mixture of polyvinyl alcohol and water, and if desired, incorporated with a polyhydric alcohol, in a tubular form, drying the tubular film to give a tubular non-stretched film having a degree of crystallization of not more than 44 % and a water content of not more than 20 % by weight, and biaxially stretching the non-stretched film at a stretching ratio of not less than 2 in each of the machine and transverse directions by means of inflation under conditions such that the film surface at the starting point of stretching is maintained at a temperature in the range from about 60° to about 150°C., the surface of the film reaching a maximum temperature at the middle portion of a stretching zone extending from said starting point to a point where the tube reaches a maximum diameter and the difference between maximum and minimum surface temperatures in the stretching zone does not vary more than about 15°C. The prepared biaxially oriented film is excellent in tensile strength, dimensional stability and impact resistance at low temperature, and has a tensile strength of not less than 1,200 Kg./cm. 2 , an elongation of 30 % to 80 % and a Young's modulus of not less than 10,000 kg./cm. 2 .

What we claim is: 
     
       1. A method for preparing polyvinyl alcohol film which comprises 1. melt-blending a mixture comprising polyvinyl alcohol having a degree of polymerization of 1,100 to 2,000 and a degree of hydrolysis of not less than 98% by mole and 70 to 110 parts by weight of water per 100 parts by weight of said polyvinyl alcohol in an extruder,   2. extruding the melted mixture through a circular slit of a die in tubular form.   
     
     
       3.  drying the extruded tubular film from both the inside and outside to provide a non-stretched film having a degree of crystallization of from 20% to 44% and a water content of not more than 20% by weight. 4. collapsing the tubular, non-stretched film,   5. heating and inflating the collapsed, non-stretched film to obtain a biaxially oriented film, said film being stretched at a ratio of not less than 2 in each of the machine and transverse directions, whereby the heating provides a temperature of the film at a starting point of stretching in a range from about 60° C to about 150° C and a maximum temperature in the middle portion of a stretching zone extending from the starting point to a point where the inflated tube reaches a maximum diameter, the difference between the maximum and minimum temperatures in the stretching zone being not more than 15° C, and   6. recovering the resulting biaxially oriented film.   
     
     
       2. The method of claim 1, in which said mixture contains at least one polyhydric alcohol selected from the group consisting of glycerin, trimethylol propane, diethylene glycol, triethylene glycol and dipropylene glycol in an amount of not more than 12 parts by weight per 100 parts by weight of said polyvinyl alcohol. 
     
     
       3. The method of claim 3, wherein the amount of said polyhydric alcohol is not more than 7 parts by weight per 100 parts by weight of said polyvinyl alcohol. 
     
     
       4. The method of claim 1 wherein said nonstretched film has a degree of crystallization of 20 to 44 % and a water content of 4 to 20 % by weight, and is stretched at a stretching ratio of 2 to 4.5 in each of the machine and transverse directions. 
     
     
       5. The method of claim 4, wherein said degree of crystallization is within the range of 25 to 44 % and said water content is within the range of 4 to 12 % by weight. 
     
     
       6. The method of claim 4, wherein said stretching ratio in each of the machine and transverse directions is within the range of 2.5 to 4.5.